Steel For Springs, Process Of Manufacture For Spring Using This Steel, And Spring Made From Such Steel

ABSTRACT

A spring steel with high fatigue resistance in air and in corrosive conditions and with high resistance to cyclic sag, having the composition in weight percent:
         C=0.45-0.70%   Si=1.65-2.50%   Mn=0.20-0.75%   Cr=0.60-2%   Ni=0.15-1%   Mo=traces-1%   V=0.003-0.8%   Cu=0.10-1%   Ti=0.020-0.2%   Nb=traces-0.2%   AI=0.002-0.050%   P=traces-0.015%   S=traces-0.015%   O=traces-0.0020%   N=0.0020-0.0110%   the balance being iron, and impurities resulting from the steel making process, where the carbon equivalent Ceq content calculated according to the formula:       

       Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %]         is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC.

The invention relates to steel making, and more specifically, the field of spring steel.

Generally, as increasing fatigue stresses are applied to springs, springs need continually increasing hardness and tensile strength. Consequently, susceptibility to fractures that begin on defects, such as inclusions or surface defects generated during spring manufacture, increases, and fatigue resistance tends to become limited. Secondly, springs used in highly corrosive environments, such as suspension springs, must have at least equivalent and preferably better fatigue properties in corrosive conditions since they use steels having higher hardness and tensile strength. Accordingly, such springs tend to fracture at the defects, immediately during the fatigue cycles in air, and more late during fatigue cycles in a corrosive medium. In particular, for fatigue in corrosive conditions, defects can begin in corrosion pits. Furthermore, with increasing applied stress, it is more difficult to improve the fatigue life in corrosive conditions or to maintain it at an equivalent level, given the fact that the effects of the concentration of stresses on the corrosion pits, on the surface defects of the springs that may be generated during spring coiling, in other steps in the manufacturing process, or in non-metallic inclusions, become more critical when spring hardness increases.

According to the prior art, documents FR-A-2740476 and JP-3474373B describe a spring steel grade with good resistance to hydrogen embrittlement and good fatigue resistance, in which inclusions of carbonitrosulfides containing at least one of the elements titanium, niobium, zirconium, tantalium or hafnium are controlled so as to have lower mean size, less than 5 μm in diameter, and to be very numerous (10,000 or more on a cutting section).

However, this type of steels leads, after quenching and tempering according to the industrial spring manufacturing process, to a hardness level of only 50 HRC or a little higher, corresponding to a tensile strength of 1700 MPa or a little higher, but not much over 1900 MPa, corresponding to a hardness of 53.5 HRC. Because of this moderate hardness level, this steel only has moderate sag resistance, steel with a higher tensile strength being needed to improve sag resistance. Accordingly, such steel does not ensure an excellent compromise between high resistance, which would be above 2100 MPa, a hardness that would be higher than 55 HRC, a high fatigue resistance in air and fatigue resistance in corrosive conditions that is at least equivalent, if not higher than that needed for springs.

The purpose of the invention is to propose means to simultaneously increase, as compared to known steels, spring hardness and tensile strength, fatigue properties in air, making fatigue resistance in corrosive conditions at least equivalent, if not higher, increase spring sag resistance and to reduce susceptibility to surface defects that can be generated during spring coiling.

With this in mind the object of the invention is a spring steel with high fatigue resistance in air and in corrosive conditions and with high resistance to cyclic sag, having the composition in weight percent:

C=0.45-0.70%

Si=1.65-2.50%

Mn=0.20-0.75%

Cr=0.60-2%

Ni=0.15-1%

Mo=traces-1%

V=0.003-0.8%

Cu=0.10-1%

Ti=0.020-0.2%

Nb=traces-0.2%

AI=0.002-0.050%

P=traces-0.015%

S=traces-0.015%

O=traces-0.0020%

N=0.0020-0.0110%

The balance being iron, and impurities resulting from the steel making process, where the carbon equivalent Ceq content calculated according to the formula:

Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %]

is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC.

The maximum size of titanium nitrides or carbonitrides observed at 1.5±0.5 mm of the surface area of a bar, or a wire rod, a slug or a spring over 100 mm² of the surface area of the section is preferably less than or equal to 20 μm, which size being the square root of the surface area of the inclusions considered as squares.

Preferably, the composition of the steel is:

C=0.45-0.65%

Si=1.65-2.20%

Mn=0.20-0.65%

Cr=0.80-1.7%

Ni=0.15-0.80%

Mo=traces-0.80%

V=0.003-0.5%

Cu=0.10-0.90%

E=0.020-0.15%

Nb=traces-0.15%

Al=0.002-0.050%

P=traces-0.010%

S=traces-0.010%

O=traces-0.0020%

N=0.0020-0.0110%

The balance being iron and impurities resulting from the steel making process.

A further object of the invention is a manufacturing process for a spring steel with high fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, according to which a liquid steel is made in a converter or an electric furnace, its composition is adjusted, it is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature; that are rolled into bars, wire rods or slugs and transformed into springs, characterized in that:

the steel is of the previous type;

after they become solid the blooms, billets or ingots have a minimum mean cooling rate of 0.3° C./s between 1450-1300° C.;

the blooms, billets or ingots are rolled between 1200-800° C. in one or two reheating and rolling cycles;

and bars, wire rods or slugs, or springs made from these, are austenitized between 850-1000° C., followed by a water quench, a polymer quench or an oil quench, and by tempering at 300-550° C., so as to deliver steel with hardness greater than or equal to 55 HRC.

A further object of the invention is springs made from such steel, and springs made of steel obtained by the previous process.

In an unexpected way, the inventors realized that a steel with the characteristics of the previously cited inclusion composition and morphology ensured, after steelmaking, casting, rolling, quenching and tempering done in specific conditions, a hardness greater than 55 HRC, while assuring excellent compromise between high endurance level to fatigue in air and to fatigue in corrosive conditions, high resistance to cyclic sag and low sensitivity to surface defects arising during manufacture of the spring.

The invention will be better understood upon reading the description that follows, given in reference to the following appended figures:

FIG. 1 which shows the results of hardness and cyclic sag tests for steels according to the invention and reference steels;

FIG. 2 which shows the results of fatigue tests in air as a function of steel hardness for steels according to the invention and reference steels;

FIG. 3 which shows the results of Charpy impact tests as a function of the steel hardness for steels according to the invention and reference steels; and

FIG. 4 which shows the results of fatigue tests in corrosive conditions as a function of steel hardness for steels according to the invention and reference steels.

The steel composition according to the invention must meet the following conditions.

The carbon content must be between 0.45% and 0.7%. After quenching and tempering carbon increases the tensile strength and hardness of the steel. If the carbon content is less than 0.45%, in the temperature range usually used to manufacture springs, no quenching and tempering treatment leads to the high strength and hardness of the steel described in the invention. Secondly, if the carbon content exceeds 0.7% preferably 0.65%, coarse and very hard carbides, combined with chromium, molybdenum and vanadium, can remain undissolved during the austenitization conducted before the quench, and can significantly affect fatigue lifetime in air, fatigue resistance in corrosive conditions and also toughness. Consequently carbon contents above 0.7% must be avoided. Preferably, it should not exceed 0.65%.

The silicon content is between 1.65% and 2.5%. Silicon is an important element that ensures, through its presence in solid solution, high levels of strength and hardness, as well as high carbon equivalent values Ceq and sag resistance. To have the tensile strength and hardness values of the steel according to the invention, the silicon content must not be less than 1.65%. Furthermore, silicon contributes at least partially to steel deoxidation. If its content exceeds 2.5%, preferably 2.2%, the oxygen content of the steel can be, by thermodynamic reaction, greater than 0.0020%, preferably 0.0025%. This involves formation of oxides of various compositions which are harmful to fatigue resistance in air. Furthermore, for silicon contents greater than 2.5%, the various combined elements such as manganese, chromium or others can segregate during solidification, after casting. This segregation is very harmful to fatigue behavior in air and to fatigue resistance in corrosive conditions. Finally, for silicon content greater than 2.5%, decarburization at the surface of bars or wire rods for springs becomes too high for the in-service properties of the springs. This is why the silicon content must not exceed 2.5%, and preferably 2.2%.

The manganese content is between 0.20% and 0.75%. In combination with residual sulfur at level of traces to 0.015%, the manganese content must be at least ten times higher than the sulfur content so as to avoid formation of iron sulfides that are extremely harmful to steel rolling. Consequently, a minimum manganese content of 0.20% is required. Furthermore, manganese contributes to solid solution hardening during the quenching of the steel as well as nickel, chromium, molybdenum and vanadium, which delivers high tensile strength and hardness values and the carbon equivalent Ceq value of the steel described in the invention. Manganese contents greater than 0.75%, preferably 0.65%, in combination with silicon, can segregate during the solidification stage, after steel making and casting. These segregations are harmful to the in-service properties and to the homogeneity of the steel. This is why the manganese content must not exceed 0.75%, and preferably 0.65%.

The chromium content must be between 0.60% and 2%, and preferably between 0.80% and 1.70%. Chromium is added to obtain, in solid solution after austenitization, quenching and tempering, high values for tensile strength and hardness, and to contribute to obtaining the carbon equivalent Ceq value, but also to increase fatigue resistance in corrosive conditions. To ensure these properties the chromium content must be at least 0.60%, and preferably at least 0.80%. Above 2%, preferably 1.7%, specific coarse, very hard chromium carbides, in combination with vanadium and molybdenum, can remain after the austenitization treatment that precedes the quench. Such carbides greatly affect the fatigue resistance in air. This is why the chromium content must not exceed 2%.

The nickel content is between 0.15% and 1%. Nickel is added to increase steel hardenability, as well as tensile strength and hardness after quenching and tempering. Since it does not form carbides, nickel contributes to steel hardening, just like chromium, molybdenum and vanadium, without forming specific coarse, hard carbides which would not be dissolved during the austenitization that precedes the quench, and could be harmful to fatigue resistance in air. It also means that the carbon equivalent can be adjusted between 0.8% and 1% in the steel according to the invention as needed. As a non-oxidizable element, nickel improves fatigue resistance in corrosive conditions. To ensure that these effects are significant, the nickel content must not be lower than 0.15%. In contrast, above 1%, preferably 0.80%, nickel can lead to overly high residual austenite content, whose presence is very harmful to fatigue resistance in corrosive conditions. Furthermore, high nickel level significantly increase the cost of the steel. For all these reasons the nickel content must not exceed 1%, preferably 0.80%.

The molybdenum content must be between traces and 1%. As for chromium, molybdenum increases steel hardenability, as well as strength. Furthermore, it has low oxidation potential. For these two reasons, molybdenum is favorable to fatigue resistance in air and in corrosive conditions. But for contents above 1%, preferably 0.80%, coarse, very hard molybdenum carbides can remain, optionally combined with vanadium and chromium, after the austenitization that precedes the quench. These particular carbides are very harmful for fatigue resistance in air. Finally, adding more than 1% molybdenum increases the cost of the steel unnecessarily. This is why the molybdenum content must not exceed 1%, and preferably 0.80%.

The vanadium content must be between 0.003% and 0.8%. Vanadium is an element that increases hardenability, tensile strength and hardness after quenching and tempering. Furthermore, in combination with nitrogen, vanadium forms a large number of fine submicroscopic vanadium or vanadium and titanium nitrides that refine the grain and increase tensile strength and hardness levels, through structural hardening. To obtain formation of submicroscopic vanadium or vanadium and titanium nitrides that refine the grain, vanadium must be present with a minimum content of 0.003%. But this element is expensive and it has to be kept at this lower limit if a compromise is sought between the cost of steel making and the grain refinement. Vanadium must not exceed 0.8% and, preferably, 0.5%, because beyond this value a precipitate of coarse, very hard vanadium-containing carbides, in combination with chromium and molybdenum, can remain undissolved during the austenitization that precedes the quench. This can be very unfavorable for fatigue resistance in air, for high values of strength and hardness in the steel according to the invention. Further, adding more than 0.8% vanadium increases the cost of the steel unnecessarily.

The copper content must be between 0.10% and 1%. Copper is an element that hardens steel when it is in solid solution after the quenching and tempering treatment. Accordingly, it can be added along with other elements that contribute in increasing the strength and hardness of the steel. As it does not combine with carbon, it hardens the steel without forming coarse, hard carbides that harm fatigue resistance in air. Form the electrochemical point of view, its passivation potential is higher than that of iron and, consequently, it favors steel fatigue resistance in corrosive conditions. To ensure that these effects are significant, the copper content must not be lower than 0.10%. In contrast, at contents of more than 1%, preferably 0.90%, copper has a very harmful influence on the behavior during hot rolling. This is why the copper content must not exceed 1%, and preferably 0.90%.

The titanium content must be between 0.020% and 0.2%. Titanium is added to form, in combination with nitrogen, preferably also carbon and/or vanadium, fine, submicroscopic nitrides or carbonitrides that refine the austenitic grain during the austenitization that precedes the quench. According, it increases the surface area of the grain boundaries in the steel, thereby reducing the quantity of unavoidable impurities that segregate in the grain boundaries, such as phosphorus. Such intergranular segregations would be very harmful to toughness and fatigue resistance in air if they are present at high concentrations per unit of surface area at the grain boundaries. Furthermore, combined with carbon and nitrogen, preferably with vanadium and niobium, titanium leads to the formation of other fine nitrides or carbonitrides producing an irreversible trapping effect for some elements, such as hydrogen formed during corrosion reactions, and which can be extremely harmful to fatigue resistance in corrosive conditions. For good efficiency the titanium content must not be lower than 0.020%. In contrast, above 0.2%, preferably 0.15%, titanium can lead to the formation of coarse, hard carbonitrides that are very harmful to fatigue resistance in air. The latter effect is yet more harmful for high levels of tensile strength and hardness in the steel according to the invention. For these reasons the titanium content must not exceed 0.2, preferably 0.15%.

The niobium content must be between traces and 0.2%. Niobium is added to form, in combination with carbon and nitrogen, extremely fine, submicroscopic precipitates of nitrides and/or carbides and/or carbonitrides that refine the austenitic grain during the austenitization that precedes the quench, especially when the aluminum content is low (0.002% for ample). Accordingly, niobium increases the surface area of the grain boundaries in the steel, and contributes to the same favorable effect as titanium as regards embrittlement of grain boundaries by unavoidable impurities such as phosphorus, whose effect is very harmful to toughness and fatigue resistance in corrosive conditions. Furthermore, extremely fine precipitates of niobium nitrides or carbonitrides contribute to steel hardening through structural hardening. However, the niobium content must not exceed 0.2%, preferably 0.15%, so that the nitrides or carbonitrides remain very fine, to ensure austenitic grain refining and to avoid cracks or splits forming during hot rolling. For these reasons the niobium content must not exceed 0.2%, preferably 0.15%.

The aluminum content must be between 0.002% and 0.050%. Aluminum can be added to complete steel deoxidation and to obtain the lowest possible oxygen contents, certainly less than 0.0020% in the steel according to the invention. Furthermore, in combination with nitrogen, aluminum contributes to refining the grain by forming submicroscopic nitrides. To ensure these two functions, the aluminum content must not be lower than 0.002%. In contrast, an aluminum content exceeding 0.05% can lead to the presence of large, isolated inclusions or to aluminates that are finer but hard and angular, in the form of long stringers that are harmful to the fatigue lifetime in air and to the cleanliness of the steel. This is why the aluminum content must not exceed 0.05%.

The phosphorus content must be between traces and 0.015%. Phosphorus is an unavoidable impurity in steel. During a quenching and tempering treatment, it co-segregates with elements such as chromium or manganese in the former austenitic grain boundaries. The result is reduced cohesion in the grain boundaries and intergranular embrittlement that is very harmful to fatigue resistance in air. These effects are even more harmful for the high tensile strengths and hardnesses required in steels according to the invention. With the aim of simultaneously obtaining high spring steel tensile strength and hardness and good fatigue resistance in air and in corrosive conditions, the phosphorus content must be as low as possible and must not exceed 0.015%, preferably 0.010%.

The sulfur content is between traces and 0.015%. Sulfur is an unavoidable impurity in steel. Its content must be as low as possible, between traces and 0.015%, and preferably 0.010% at most. Accordingly, we wish to avoid the presence of sulfides that are unfavorable to fatigue resistance in corrosive conditions and fatigue resistance in air, for high values of strength and hardness in the steel according to the invention.

The oxygen content must be between traces and 0.0020%. Oxygen is also an unavoidable impurity in steel. In combination with deoxidizing elements, oxygen can lead to isolated, coarse, very hard, angular inclusions appearing, or to inclusions that are finer but in the form of long stringers which are very harmful to fatigue resistance in air. These effects are even more harmful at the high tensile strengths and hardnesses of the steels according to the invention. For these reasons, to ensure a good compromise between high tensile strength and hardness and high fatigue resistance in air and in corrosive conditions in the steel according to the invention, the oxygen content must not exceed 0.0020%.

The nitrogen content must be between 0.0020% and 0.0110%. The nitrogen must be controlled in this range so as to form, in combination with titanium, niobium, aluminum or vanadium, a sufficient number of very fine submicroscopic nitrides, carbides or carbonitrides that refine the grain. Accordingly, to do so the minimum nitrogen content must be 0.0020%. Its content must not exceed 0.0110% so as to avoid forming coarse, hard titanium nitrides or carbonitrides larger than 20 μm, observed at 1.5 mm±0.5 mm from the surface of the bars or wire rods used to manufacture the springs. This position is the place that is most critical as regards the fatigue loading of the springs. Indeed, such large nitrides or carbonitrides are very unfavorable to fatigue resistance in air for high strength and hardness values for steels according to the invention, given the fact that during the tests on fatigue in air, these springs fractured at the location of such large inclusions that were located precisely in the cited area of the surface of the springs, when these inclusions were present.

To estimate the size of the titanium nitrides and carbonitrides, we consider the inclusions as squares and we suggest that their size is equal to the square root of their surface area.

A manufacturing process for springs according to the invention will now be described.

A non-limiting steel making process that conforms to the invention is as follows. Liquid steel is produced either in a converter, or in an electric furnace, then undergoes a ladle metallurgy treatment during which alloy elements are added and deoxidation is performed, and in general all secondary metallurgy operations delivering a steel having the composition according to the invention and avoiding formation of sulfide or “carbonitrosulfide” complexes of elements such as titanium and/or niobium and/or vanadium. To avoid formation of such coarse precipitates during steel making, the inventors have discovered, in an unexpected way, that the contents of the various elements, in particular those of titanium, nitrogen, vanadium and sulfur, must be carefully controlled in the previously cited limits. After the process that has just been described the steel is cast in the form of blooms or billets, or into ingots. But to completely avoid forming, or to avoid forming as much as possible, coarse titanium nitrides or carbonitrides during and after the solidification of these products, we have found that the mean cooling rate of these products (blooms, billets or ingots) must be controlled so as to be 0.3° C./s or higher between 1450-1300° C. When we operate in these conditions during the solidification and cooling stages, we observe in an unexpected way that the size of the coarsest titanium nitrides or carbonitrides observed on the springs is always less than 20 μm. The location and size of these titanium precipitates will be discussed hereinafter.

When they have returned to room temperature, products having the precise composition according to the invention (blooms, billets or ingots) are next reheated and rolled between 1200-800° C. into the form of wire rods or bars in a single or double heating and rolling process. So as to obtain the properties of the steel that is specific to the invention, the bars, rods, slugs, or even springs produced from these bars or wire rods, are next subjected to a water quench treatment, a polymer quench or an oil quench after austenitization in a temperature range from 850-1000 C, so as to obtain a fine austenitic grain where there are no grains coarser than 9 on the ASTM grain size scale. This quenching treatment is then followed by a tempering treatment specifically performed between 300-550° C., that delivers the high levels of tensile strength and hardness required for the steel, and avoids firstly a microstructure that would lead to embrittlement during tempering, and secondly, overly high residual austenite. We found that embrittlement during tempering and an overly high level of residual austentite are extremely harmful to fatigue resistance in corrosive conditions of the steel according to the invention. In the case where the springs are manufactured from bars that have not been heat treated or from wire rods or slugs made from such bars, the abovementioned treatments (quenching and tempering) must be performed on the springs themselves under the abovementioned conditions. In the case where the springs are manufactured from using cold forming, these heat treatments can be done on the bars, wire rods or slugs made from these bars before manufacturing the spring.

It is well known that the hardness of steel depends not only on its composition, but also on the quenching temperature that it was subjected to. It must be understood that for all the compositions of the invention, it is possible to find quenching temperatures in the industrial range of 300-550° C. that deliver the minimum targeted hardness of 55 HRC.

Since nitrides and carbonitrides are very hard, their size as previously defined does not change at all during the steel transformation steps. Therefore it is not important whether it is measured on the intermediate product (bar, wire rod or slug) which will be used to manufacture the spring or on the spring itself.

The invention delivers spring steels that can combine high hardness and tensile strength that are an improvement over the prior art, as well as improved fatigue properties in air and sag resistance, fatigue properties in corrosive conditions at least equivalent to those of known steels for this use, or even better, and lesser susceptibility to concentrations of stresses produced by surface defects that can form during spring manufacture, through addition of microalloyed elements, a reduction in residual elements and control of the analysis and production route of the steel.

The invention is now illustrated using examples and reference examples. Table 1 shows steel compositions according to the invention and reference steels. The carbon equivalent Ceq is given by the following formula:

Ceq=[C]+0.12 [Si]+0.17 [Mn]−0.1 [Ni]+0.13 [Cr]−0.24 [V]

where [C], [Si], [Mn], [Ni], [Cr] and [V] represent the content of each element in weight percent.

TABLE 1 Chemical compositions of the tested steels (in %) C Si Mn Ni Cr V Ti Cu Mo Nb P S Al N O Ceq Steel of the 0.48 1.82 0.21 0.15 1.48 0.204 0.072 0.20 0.02 0 0.006 0.006 0.034 0.0051 0.0007 0.86 invention 1 Steel of the 0.58 1.79 0.22 0.15 0.98 0.216 0.073 0.20 0.03 0 0.006 0.008 0.032 0.0051 0.0007 0.89 invention 2 Steel of the 0.59 1.80 0.22 0.15 0.99 0.212 0.025 0.20 0.03 0.022 0.007 0.008 0.032 0.0066 0.0008 0.91 invention 3 Steel of the 0.48 2.10 0.21 0.70 1.50 0.152 0.069 0.51 0.03 0 0.005 0.005 0.032 0.0042 0.0008 0.86 invention 4 Steel of the 0.54 1.81 0.23 0.34 1.25 0.098 0.077 0.42 0.02 0 0.006 0.008 0.031 0.0041 0.0007 0.90 invention 5 Reference 0.60 1.73 0.88 0.08 0.20 0.154 0.002 0.19 0.03 0.020 0.010 0.019 0.002 0.0084 0.0010 0.94 steel 1 Reference 0.40 1.79 0.17 0.53 1.04 0.166 0.064 0.20 0.01 0 0.013 0.004 0.020 0.0034 0.0011 0.69 steel 2 Reference 0.48 1.45 0.89 0.11 0.47 0.136 0.002 0.19 0.02 0 0.011 0.013 0.003 0.0062 0.0010 0.82 steel 3

Table 2 shows the hardness values obtained for steels according to the invention and reference steels as a function of the quenching temperature that was used.

TABLE 2 Hardness and tensile strength as a function of the tempering temperature Quenching HRC Quenching HRC temperature hard- temperature hard- (° C.) ness (° C.) ness Steel of the invention 1 350 56.9 400 55.3 Steel of the invention 2 350 58.5 400 57.1 Steel of the invention 3 350 59.0 400 57.2 Steel of the invention 4 350 56.7 400 55.6 Steel of the invention 5 350 57.6 400 55.8 Reference steel 1 350 57.9 400 55.1 Reference steel 2 350 54.2 400 52.5 Reference steel 3 350 54.8 400 51.3

Table 3 shows the maximum size of the inclusions of titanium nitride or carbonitrides observed at 1.5 mm from the surface of steels according to the invention and reference steels, as previously defined. We have also reported the titanium contents of the various steels.

The maximum size of such titanium nitride or carbonitride inclusions is determined as follows. On a section of bar or wire rod coming from a given steel cast, a surface area of 100 mm² is examined at a point located 1.5 mm±0.5 mm below the surface of the bar or wire rod. After the observations, the size of the titanium nitride or carbonitride inclusion having the largest surface area is determined by considering that the inclusions are squares and that the size of each of these inclusions, including the inclusion having the largest surface area, is equal to the square root of the surface area. All the inclusions are observed on a section of bar or wire rod for springs, and the observations are performed on 100 mm² of each section. The steel cast conforms to the invention when the maximum size of the abovementioned inclusions observed on 100 mm² at 1.5 mm±0.5 mm under the surface is less than 20 μm. The corresponding results obtained on steels according to the invention and reference steels are given in table 3.

As regards the reference tests 1 and 3, their titanium content is practically nil and no nitrides and carbonitrides are observed.

TABLE 3 Maximum size of the largest titanium nitride or carbonitride inclusions at 1.5 mm from the surface of the samples Size of the largest nitride or carbonitride observed on Ti (%) 100 mm² (μm) Steel of the invention 1 0.072 11.8 Steel of the invention 2 0.073 12.4 Steel of the invention 3 0.025 13 Steel of the invention 4 0.069 11.9 Steel of the invention 5 0.077 14.1 Reference steel 1 0.002 — Reference steel 2 0.064 20.8 (first exam) Reference steel 2 0.064 29 (second exam) Reference steel 3 0.002 —

We did not measure the size of the inclusions with reference steels 1 and 3, since their titanium content was low and did not conform to the invention: the result would not have been significant.

Samples for fatigue testing were taken from bars, and the final diameter of the samples was 11 mm. Preparation of the samples for fatigue testing included rough machining, austenitization, oil quenching, tempering, grinding and shot-peening. These samples were torsion-fatigue tested in air. The shear stress applied was 856±494 MPa and the number of cycles to fracture was counted. The tests were stopped after 2.10⁶ cycles if the samples had not broken.

Samples for fatigue testing in corrosive conditions were taken from bars, and the final diameter of the samples was 11 mm. Preparation of the samples for fatigue testing included rough machining, austenitization, oil quenching, tempering, grinding and shot-peening. These samples were tested for fatigue in corrosive conditions, i.e. corrosion was applied at the same time as a fatigue load. The fatigue load was a shear stress of 856±300 MPa. The corrosion applied was cyclic corrosion in two alternating stages:

one stage being a wet stage, with spraying of a 5% NaCl solution for 5 minutes at 35° C.;

one stage being a dry stage without spraying, for 30 minutes at a temperature of 35° C.

The number of cycles to fracture was considered to be the fatigue life in corrosive conditions.

Sag resistance was determined using a cyclic compression test on cylindrical samples. The sample diameter was 7 mm and their height was 12 mm. They were taken from steel bars.

Preparation of the samples for sag testing included rough machining, austenitizing, oil quenching, tempering and final fine grinding. The height of the sample was measured precisely before starting the test by using a comparator having 1 μm precision. A preload was applied so as to simulate spring presetting, this presetting being a compression stress of 2200 MPa.

Then the fatigue load cycle was applied. This stress was 1270±730 MPa. The height loss in the sample was measured for a number of cycles, up to 1 million. At the end of the test the total sag was determined by a precise measurement of the remaining height compared to the initial height, sag resistance being better when the reduction in height, as a percentage of the initial height, was lower.

The results of the fatigue tests, fatigue tests in corrosive conditions and sag on steels according to the invention and reference steels are given in table 4.

TABLE 4 Results of fatigue, fatigue in corrosive conditions and sag tests Fatigue life- time in corrosive Tensile Fatigue life- conditions HRC strength time (number (number of Sag hardness (MPa) of cycles) cycles) (%) Steel of the 56.7 2129 1742967 192034 0.025 invention 1 Steel of the 56.4 2106 >2000000 138112 0.01 invention 2 Steel of the 56.5 2118 >2000000 135562 0.015 invention 3 Steel of the 56.9 2148 >2000000 202327 0.025 invention 4 Steel of the 57.0 2156 >2000000 139809 0.025 invention 5 Reference 56.7 2131 514200 96672 0.03 steel 1 Reference 53.8 1898 217815 241011 0.10 steel 2 Reference 55.6 2062 301524 150875 0.075 steel 3

From these tables, we see that the various reference steels are unsatisfactory, in particular for the following masons.

Reference steel 1, in particular, has sulfur content that is too high for good compromise between fatigue resistance in air and the content for fatigue in corrosive conditions. Furthermore, its manganese content is too high, which leads to segregations that are harmful for the homogeneity of the steel and fatigue resistance in air.

Reference steel 2 has too low carbon content and carbon equivalent to ensure high hardness. Its tensile strength is too low for good fatigue resistance in air.

Reference steel 3, in particular, has silicon content that is too low for good sag resistance and also good fatigue resistance in air.

Sag resistance is higher for the steels of the invention than for reference steels, as FIG. 1 shows, where it is clear that according to the abovementioned sag measurements, the values for sag are at least 32% lower for the worst case of the steels of the invention (steel of the invention 1) as compared to the best ease of the reference steels (reference steel 1).

The fatigue lifetime in air is clearly higher for the steels of the invention as compared to the reference steels. This is due to the increased hardness, as FIG. 2 shows, but increased hardness is not enough. In fact, generally, steels with high hardness are more susceptible to defects, such as inclusions and surface defects as the hardness increases. Accordingly, steels according to the invention are less susceptible to defects, in particular to coarse inclusions such as titanium nitrides or carbonitrides, given that the invention prevents such large inclusions appearing. As table 3 shows, the largest inclusions found in steels according to the invention do not exceed 14.1 μm, where inclusions larger than 20 μm are found in reference steel 2. Furthermore, lower susceptibility to surface defects such as those that arise during spring manufacture or other operations when steels of the invention are used can be illustrated by strength tests performed on steels of the invention and reference steels having undergone a heat treatment and having hardness of 55 HRC or higher, see FIG. 3. The values measured during Charpy impact tests on the steels of the invention (where the sample notch simulates a concentration of stresses like other concentrations of stresses that we can find on surface defects produced during the manufacture of the spring or other operations) are higher than those measured on the reference steels. This shows that the steels according to the invention are less susceptible to concentrations of stresses on defects than reference steels according to the prior art.

We know that increasing hardness reduces fatigue resistance in corrosive conditions. Accordingly, it seems that steels according to the invention have the advantage that their fatigue resistance in corrosive conditions is higher than that of reference steels according to the prior art, and in particular hardness greater than 55 HRC as FIG. 4 shows.

Accordingly, the invention delivers higher hardness with a good compromise between fatigue lifetime in air and sag resistance, which are greatly increased, and fatigue lifetime in corrosive conditions which is better than those of reference steels according to the prior art. Furthermore, lesser susceptibility to possible surface defects, in particular those generated during spring manufacture or other operations, is also obtained. 

1: A spring steel composition characterized by high fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, said composition comprising in weight percent: C=0.45-0.70% Si=1.65-2.50% Mn=0.20-0.75% Cr=0.60-2% Ni=0.15-1% Mo=traces-1% V=0.003-0.8% Cu=0.10-1% Ti=0.020-0.2% Nb=traces-0.2% Al=0.002-0.050% P=traces-0.015% S=traces-0.015% O=traces-0.0020% N=0.0020-0.0110% the balance being iron, and impurities resulting from the steel making process, wherein the carbon equivalent (Ceq) content calculated according to the formula: Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %] is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC. 2: The sing steel according to claim 1, characterized in that the maximum size of titanium nitrides or carbonitrides observed at 1.5±0.5 mm of the surface area of a bar, a wire rod, a slug or a spring over 100 mm² of the surface area of the section is less than or equal to 20 μm, said size being the square root of the surface area of the inclusions considered as squares. 3: The spring steel according to claim 1, characterized in that its composition comprises: C=0.45-0.65% Si=1.65-2.20% Mn=0.20-0.65% Cr=0.80-1.7% Ni=0.15-0.80% Mo=traces-0.80% V=0.003-0.5% Cu=0.10-0.90% Ti=0.020-0.15% Nb=traces-0.15% AI=0.002-0.050% P=traces-0.010% S=traces-0.010% O=traces-0.0020% N=0.0020-0.0110% the balance being iron and impurities resulting from the steel making process. 4: A manufacturing process for a spring steel with high fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, according to which a liquid steel is made in a converter or an electric furnace, its composition is adjusted, it is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature; that are rolled into bars, wire rods or slugs for transformation into springs, characterized in that: the steel is the type according to claim 1: the blooms, billets or ingots after they become solid have a minimum mean cooling rate of 0.3° C./s between 1450-1300° C.; said blooms, billets or ingots are rolled between 1200-800° C. in one or two reheating and rolling cycles; and bars, wire rods or slugs, or springs made from these, are austenitized between 850-1000° C., followed by a water quench, a polymer quench or an oil quench, and by tempering at 300-550° C., so as to deliver steel with a hardness greater than or equal to 55 HRC. 5: A spring, characterized in that it is made of a steel according to claim
 1. 6: A spring according to claim 5, characterized in that it is made of a steel comprising in weight percent: C=0.45-0.70% Si=1.65-2.50% Mn=0.20-0.75% Cr=0.60-2% Ni=0.15-1% Mo=traces-1% V=0.003-0.8% Cu=0.10-1% Ti=0.020-0.2% Nb=traces-0.2% Al=0.002-0.050% P=traces-0.015% S=traces-0.015% O=traces-0.0020% N=0.0020-0.0110% the balance being iron, and impurities resulting from the steel making process, wherein the carbon equivalent (Ceq) content calculated according to the formula: Ceq %=[C %]+0.12 [Si %]+0.17 [Mn %]−0.1 [Ni %]+0.13 [Cr %]−0.24 [V %] is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55 HRC, wherein said steel is made in a converter or an electric furnace, its composition is adjusted and is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature and are rolled into bars, wire rods or slugs for transformation into springs, characterized in that: the blooms, billets or ingots after they become solid have a minimum means cooling rate of 0.3° C./s between 1450-1300° C.; said blooms, billets or ingots are rolled between 1200-800° C. in one or two reheating and rolling cycles; and bars, wire rods or slugs, or springs made from these, are austenitized between 850-1000° C., followed by a water quench, a polymer quench or an oil quench, and by tempering at 300-550° C., so as to deliver steel with a hardness greater than or equal to 55 HRC. 